Review of Radiation Therapy Physics: A syllabus for teachers ... - IRSN

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Chapter 11. Computerized Treatment Planning Systems for External Beam radiotherapy11.3.7. Biological modelingDistributions modeled on biological effects may in the future become more clinically relevantthan those based upon dose alone. Such distributions will aid in predicting both the TumourControl Probability (TCP) and the Normal Tissue Complication Probability (NTCP). Thesealgorithms can account for specific organ dose response, and aid in assessing dosefractionation and volume effects. Patient specific data can be incorporated in the biologicalmodel to help predict individual dose-response to a proposed treatment regime.11.4. DATA ACQUISITION AND ENTRY11.4.1. Machine data• Prior to entering radiation data into the TP systems, the various mechanicalcomponents of the treatment machines must be obtained so that the TP systemmodel of the machine agrees with the possible mechanical motions and limits ofthe machine. The gantry, couch and collimator rotation conventions used in aparticular institution must be described accurately, and the angle convention fullyunderstood. The TP system must also be able to distinguish between jaw pairs andaccurately model the limits of the jaw over-travel.• Static and virtual wedge use by the TP system will be limited to field sizes thatcorrespond to the maximum field setting in both the transverse and longitudinaldirections. Dynamic wedge use may also be limited by the jaw over-travel and bythe maximum dose rate available on the linac. Specific files that are used by thelinac to generate jaw movements, such as Segmented Treatment Tables (STT),may also be used directly by the TP system.• The TP system models the MLC leaf design and leaf motion. Blocking trays mayreside at several distances, and this is accounted for by the TP system forpenumbra generation. Blocks with straight or tapered edges may be modeledseparately.• Linacs that are capable of producing IMRT fields may do so via step-and-shoot orfully dynamic techniques. For these types of treatments the TP system requiresdata regarding maximum leaf speed, characteristics of the maximum rise in thebeam-on time, and information on maximum dose rates.• Missing tissue compensators and dose compensators can be calculated by the TPsystem, and physical data related to the attenuation coefficients of materials usedto fabricate physical compensators is required.• Electron cone design varies from one linac manufacturer to another. The TPsystem may require information regarding the distance from the cone to thenominal SSD as well as the external dimensions of the electron cone to producerooms-eye-view so as to avoid potential patient-machine collisions.326
Review of Radiation Oncology Physics: A Handbook for Teachers and Students11.4.2. Beam data acquisition and entry• The beam data required by the TP system must be well understood. This isespecially true when acquiring beam data from the treatment units. Specialconsideration must be given to the geometry of the radiation detector (typicallyionisation chambers or diodes), and any geometrical correction factors that mustbe applied to the data. Beam data is often smoothed and re-normalized bothfollowing measurement and prior to data entry into the TP computer.• Typical photon beam data sets include central axis PDDs and off-axis ratios(profiles) for open and wedged fields for a range of square fields. Diagonal fieldprofiles to account for radial and transverse open beam asymmetry and wedgedfield lateral profiles to account for the variation in wedge hardening off axis mayalso be required. In the case of diagonal profiles it may only be possible toacquire half field scans depending upon the size of the water tank.• Penumbra may be fitted to, or extracted from, measured data. In either case it isimportant that scan lengths be of sufficient length, especially for profiles at largedepths where field divergence can become considerable.• Relative or absolute field size factors are required for TP systems. These valuesare used both in treatment time calculations and in the calculation of dosedistributions involving dynamic beams (dynamic wedge, dynamic MLC, etc).Particular care must be taken with respect to the reference field size, referencedepth and nominal SSD, as these will have a global effect on time and MonitorUnit calculations. Central axis wedge factors, tray factors and other accessoryfactors (normally the ratio of dose with and without the accessory) are alsorequired by the TP system.• Measured beam data relevant to the MLC can include transmission through theleaf, inter-leaf transmission between adjacent leaves, and intra-leaf transmissionoccurring when leaves from opposite carriage banks meet end-on.• Beam measurement for electrons is more difficult than for photons because of thecontinuously decreasing energy of the beam with depth. Electron beam datameasured with ionisation chambers must be first converted to dose with anappropriate method, typically using a look-up table of stopping power ratios.Measurements with silicon diodes are often considered to be tissue equivalent andgive a direct reading in dose.• Monte Carlo TP systems require accurate information concerning the geometryand composition of linac beam-line components, such as the waveguide exitwindow, target, flattening filter, scattering foil, transmission ionisation chamber,jaws, MLC, blocks and trays, and any other items the electron or photon beam islikely to encounter.• Beam data acquired from a linac may be entered manually using a digitizer tabletand tracing stylus. Hardcopy of beam data is used, and it is important that boththe beam data printout and the digitizer be routinely checked for calibration.327
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